As shown in FIGS. 1-3, an axial pin tumbler lock of the prior art comprises a lock 10 and a key 20. The lock 10 comprises: a housing 11 provided therein with a first axial hole 111 and a second axial hole 112; an outer tumbler pin seat 12 received in the second axial hole 112 and provided with a plurality of tumbler pin through holes 121 and with a lock core 122; a plurality of outer tumbler pins 13 of various lengths and received slidably in the tumbler pin through holes 121; an inner tumbler pin seat 14 received in the second axial hole 112 and provided with a third axial hole 141 dimensioned to receive therein the lock core 122; a plurality of inner tumbler pins 15; and a plurality of springs 16. The key 20 comprises a handle 21 and a cylinder 22 having an outer diameter corresponding to an inner diameter of the first axial hole 111. The cylinder 22 is provided at the free end thereof with a plurality of retaining slots 23 which are corresponding in location to the outer tumbler pins 13 and are of different lengths.
In operation, the cylinder 22 of the key is inserted into the first axial hole 111 of the lock 10 such that the retaining slots 23 force the outer tumbler pins 13 and the inner tumbler pins 15 to move inwards so as to cause the junctions between the outer tumbler pins 13 and the inner tumbler pins 15 to be corresponding to the junction between the inner tumbler pin seat 14 and the outer tumbler pin seat 12. In the meantime, the handle 21 of the key 20 is so rotated as to force the cylinder 22 to actuate the lock core 122 via the retaining slots 23 and the outer tumbler pins 13.
Such a prior art axial pin tumbler lock as described above is inherently defective in design in that the outer tumbler pins 13 are partially exposed in the first axial hole 111 in order to be received in the retaining slots 23 of the key 20, thereby permitting an unauthorized person to see through one-end of the lock 10 the precise location of each of the outer tumbler pins 13, as shown in FIG. 3.